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text of picture standards

Picture Standards

This page was developed to explain the difference between the video formats.

Firstly, a history lesson. 20 years ago, this article would never have been written. There was no need. People only had a TV set, and if they were really lucky, a VCR. The VCR was connected to the TV via the only means possible, a RF antenna cable.

This was fine for most people, and delivered a pretty good picture and sound.

Then came HI-FI and DVD and later, Digital TV and then high definition digital TV (HDTV)..

TV sets had to change to accommodate these things, and with that, a huge range of plugs and standards emerged. Before we can talk about the different standards, we first must cover what actually comes out of a video camera, as that is what creates the pictures in the first place.

A video camera records images in the three colours. This is called RGB, which stands for Red, Green and Blue - co-incidentally the three colours used to record pictures. You can create every possible colour by using different amounts of these colours. But you also need additional information to place the colours in the right spot on your television. So a horizontal shift and a vertical shift is included in the data, giving 5 channels.

This is a pure digital format, and is now delivered to TV sets via either a DVI or HDMI cable. There is little or no compression employed in putting the picture on screen, and therefore you get all the detail. These formats are high definition formats, which means that you need a high definition display to fully take advantage of the increase in picture quality.

But this format requires a huge amount of bandwidth. To get over this and to ensure that programs are archived in a reasonable quality, studios convert the RGB signal into a component signal.

This is represented by YPbPr or YCbCr. The Y part of both these terms is the black and white information and the b and r parts are the colour information. RGB is not the same as YP or YC, but it is easy to convert between the two. There are 3 channels of information in this system.

YP is an analogue format, and YC is a digital format.

Many Set Top Boxes can output in RGB through the 21 pin SCART connector. SCART will be explained at the bottom of this article.

Now, not all TVs have RGB inputs, although these are becoming more popular. Almost all set top boxes have RGB out.

S-VHS is the first step in down conversion. The S-VHS signal is made up of the Y part of the signal, the light and dark part, and also a combination of the colour portion of the RGB signal. So there are two separate components to a S-VHS signal, which is a combination of three original signals. This represents a loss of colour detail, but it is not a huge loss of quality. S-VHS is actually not a connector standard, the technical term is Y/C - but everyone calls it S-VHS. Y is the brightness, and C is the colour component.

The last down conversion is a composite signal. It is called this because it is a combination of the black and white signal as well as the colour component of the signal. This provides a significantly reduced picture quality, as all the colour information is carried on one wire.

These problems can be seen in the following ways.
When you are watching hard edge objects like text, captions or solid geometric shapes you will notice that the edges appear to 'move' constantly. This is commonly called dot crawl. This does not occur with S-VHS, as it contains more information for the television to decode, and therefore it is sure where to put the dots. No more crawl.

Another problem is when there are two or more fine lines spaced close together. What happens is these two or more lines blend together and you get an incorrect colour displayed. This can be demonstrated with some black and white movies, where you can sometimes see purple fragments of incorrect colour.

A third problem is that once a signal is made composite, you can not 'reverse engineer' the signal. It will be degraded forever.

So what does this mean to a Digital Set Top Box user?

If your television will support it, use RGB as a means to deliver the pictures to your television. This will provide you with the best possible picture quality.

If your television supports component, then use this - it's almost identical to RGB in quality, and you'll get a very good picture on your set.

If your television doesn't support RGB or Component, S-VHS is not too far behind in picture quality, and will prove to be a significant upgrade from a composite signal.

Of course, if your television will only support a composite signal, you are stuck. All is not lost though, as you can always consider purchasing high quality RCA cables to deliver noticeable picture quality increases. These cables are available on our web site. www.australianhometheatrecables.com.au

All of these signals are delivered through the SCART sockets on your Set Top Box. You can select the output type via the settings menu, look for SCART OUTPUT and then choose your signal. If you are connecting to a television with a SCART input, then you need a SCART to SCART cable. You may also have to adjust your TV to receive the signal you choose.

But what is SCART?

SCART is a European standard plug that is quickly gaining popularity here in Australia. SCART delivers multiple signals through 21 pins.

Female socket on your digital set top box:

 

 

Male plug (the end of the SCART cable):

 

 

The pins are all identified below. You will note there are also two data channels, these are to allow data transmission between two SCART appliances.

1 AOR Audio Out Right

2 AIR Audio In Right

3 AOL Audio Out Left + Mono

4 AGND Audio Ground

5 B GND RGB Blue Ground

6 AIL Audio In Left + Mono

7 B RGB Blue In

8 SWTCH Audio/RGB switch / 16:9

9 G GND RGB Green Ground

10 CLKOUT Data 2: Clock pulse Out

11 G RGB Green In

12 DATA Data 1: Data Out

13 R GND RGB Red Ground

14 DATAGND Data Ground

15 R RGB Red In / Chrominance

16 BLNK Blanking Signal

17 VGND Composite Video Ground

18 BLNKGND Blanking Signal Ground

19 VOUT Composite Video Out

20 VIN Composite Video In / Luminance

21 SHIELD Ground/Shield

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